Machine designers rely on rolling element bearings or fluid film bearings (hydrostatic bearings or hydrodynamic bearings) to support the shafts of rotary machines. The type of bearing selected for a given application depends in part on the shaft diameter and rotational speed and on the dynamic behavior of the shaft. Designers use "DN", the numerical product of shaft diameter expressed in millimeters and rotational speed expressed in revolutions per minute, as a rough guide for bearing selection. Hydrostatic bearings are often favored for machines in which the shaft dynamics are a concern. For example hydrostatic bearing are often favored when the DN parameter exceeds about 2.5 million and the fluid comprising the fluid film is a low viscosity fluid. Hydrostatic bearings are also often preferred for machines in which the DN parameter is below about 300,000 and the fluid is a high viscosity fluid.
A hydrostatic bearing includes a collar with a cylindrical, radially inner load bearing surface. An array of shallow pockets extends circumferentially along the bearing surface. The pocket array is axially bordered by forward and aft edge lands, and each pocket is circumferentially separated from its two neighboring pockets by interpocket lands. The pocket array is axially centered on the bearing surface so that the axial lengths of the two edge lands are approximately equal. A fluid injection passage opens into each pocket to connect the pockets to a supply of pressurized fluid. When the bearing is installed in the rotary machine, the bearing collar circumscribes the shaft to define a narrow film annulus whose radial dimension is typically about 0.1 percent of the shaft diameter.
In operation, the pressurized fluid is continually injected into the pockets by way of the injection passages. The pressurized fluid flows into and fills the film annulus to form a load supportive fluid film for supporting the shaft. The fluid then flows axially out of the annulus and into relatively low pressure vent regions axially adjacent to each edge land.
One shortcoming of a conventional hydrostatic bearing becomes apparent if the vent pressures are substantially unequal. Under these conditions, a greater quantity of fluid flows toward the low pressure vent region than toward the high pressure vent region, compromising the stiffness (load supporting qualities) of the fluid film on the high pressure side of the bearing. Common practice is to compensate for the unequal vent pressures by employing pockets and radial injection passages that are offset toward the high pressure vent region. However this practice is not entirely satisfactory since it fails to adequately address the problem of diminished film stiffness and can degrade the vibration damping qualities of the fluid film. Moreover, the offset configuration can increase the cross coupled stiffness of the bearing and shaft assembly, leading to self excited, unstable shaft motion.
A second shortcoming of a conventional hydrostatic bearing is evident even when the vent pressures are not substantially unequal. The rotation of the shaft imparts a substantial tangential velocity component to the fluid in the film annulus. The accompanying fluid dynamic drag elevates the cross coupled stiffness of the bearing and shaft assembly and therefore establishes a threshold rotational speed above which the shaft is susceptible to self excited, unstable rotary motion. One possible way to increase the stability of the rotor system is suggested in a 1967 paper entitled "Bearings with a Tangential Gas Supply" by Ales Tondl. Tondl advocates the use of obliquely oriented injection passages ("nozzles") so that the injected fluid enters the pockets with a tangential directional component. Tondl reports having tested a bearing configuration with eight pockets, each featuring two such injection passages. Tondl concludes, based on theoretical analysis supported by experimental evidence, that susceptibility to self excited instabilities is diminished if the fluid is introduced into the pockets with a tangential directional component opposite to the direction of shaft rotation. A hydrostatic bearing with stability enhancing, tangentially directed supply passages is also disclosed in U.K. Patent 1,149,425. A further improved hydrostatic bearing is disclosed in U.S. Pat. No. 5,433,528. That patent acknowledges the benefits of tangentially directed fluid injection opposite to shaft rotation. The patent illustrates a bearing with tangentially directed supply passages and a pair of diametrically opposed axial grooves that enhance bearing stability. However, despite the above described advances in improving the stability of hydrostatic bearings, engineers and designers continually seek ways to achieve stable rotor operation at higher rotational speeds.